1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing unit which fixes a heated and melted unfixed toner onto a sheet carrying a toner image while passing the sheet of paper through a nip defined between a pair of heating rollers or a heating belt and a roller.
2. Description of the Related Art
In this type of image forming apparatus, in order to meet demands such as shortening the warm-up time of a fixing unit and saving energy, attention has recently been drawn to a belt method capable of operating with a smaller amount of heat capacity (e.g., refer to Japanese Patent Laid-Open Publication No. H06-318001). In recent years, an electromagnetic induction heating method (IH) capable of rapid heating or high-efficient heating has also been notable, and taking into account saving energy when fixing a color image, the image forming apparatus employing the combination of the electromagnetic induction heating and belt methods have been put on the market. The combination of the belt method and the electro-magnetic induction heating has advantages in that a coil can be easily laid out and cooled and a belt can be directly heated. These and other advantages prompt an electromagnetic inductor to be arranged outside of the belt (so-called external IH type).
In the electro-magnetic induction heating method, various arts have been developed for the purpose of preventing an excessive temperature rise in a non-sheet conveyed region in accordance with the width (conveyed-sheet width) of a sheet conveyed through a fixing unit. Particularly, a means for the different sizes of sheets in the external IH is described in the following prior arts, for example, Japanese Patent Laid-Open Publication No. 2003-107941 and Japanese Patent Laid-Open Publication No. 2006-120523).
In a first prior art (Japanese Patent Laid-Open Publication No. 2003-107941 (FIGS. 2 and 3)), a magnetic member is divided into several parts and arranged in a conveyed-sheet width direction, and some of the divided parts of the magnetic member are moved close to and apart from an excitation coil in accordance with the width (conveyed-sheet width) of a conveyed sheet. In this case, some of the divided parts of the magnetic member are moved apart from the excitation coil in a non-sheet conveyed region, thereby lowering the heat-generation efficiency in the non-sheet conveyed region to make the generated-heat quantity smaller than that in a minimum sheet conveyed region for a sheet of a minimum width.
In a second prior art (Japanese Patent Laid-Open Publication No. 2006-120523), a magnetic shielding plate having a curved-surface is formed in advance with a plurality of steps in the longitudinal directions thereof, and these steps form an area for passing magnetism and an area screening out magnetism in the width direction of a sheet. Therefore, when the size of a sheet is changed, the magnetic shielding plate is turned in accordance with the conveyed-sheet width, thereby screening out magnetism in a non-sheet conveyed region to suppress an excessive rise in the temperature of a heated roller or the like.
However, the first prior art has the problem of requiring a wider motion space for the magnetic member, thereby making the whole apparatus larger.
In the second prior art, the positions of the steps formed beforehand in the shielding plate determine the shielding area and the non-shielding area, thereby making it difficult to handle sheets of paper having many different sizes. Besides, if the steps are formed in the direction in which the shielding plate turns, then the turning angle as a whole is restricted to hinder enlarging each step (e.g., a turning angle of approximately 15°-30°), thereby reducing the quantity of screened-out magnetism and making it impossible to suppress the generated-heat quantity sufficiently.